The invention pertains to compressed air drivers, and particularly pertains to a valve arrangement for such apparatus.
Conventional valve arrangements for compressed air driven apparatus utilize head valves in which the valve member is a differential piston which is arranged coaxially with respect to the working cylinder and above the stroke chamber thereof. The differential piston closes the work chamber with respect to the source of compressed air when the piston is in a lower or closed position, and when the piston is an upper position the working chamber communicates with an outlet or exhaust port. The larger of the control surfaces of the control piston is selectively biased with pressure or connected to the atmosphere by a valve which is automatic or manually controlled. Such a valve arrangement is known, and is shown in German Pat. No. 1,285,959. The disadvantage of this type of valve arrangement is that it is not free from overlapping valve operations such that communication of the compressed air reservoir with the working chamber takes place before the exhaust port is completely closed, and the working chamber is in communication with the exhaust port before the communication with the compressed air source is terminated. The operation of a valve arrangement which is not free from such valve overlapping operation results in air pressure losses due to inefficient use of compressed air, and high noise levels are produced resulting from the momentary escape of compressed air directly through the exhaust port.
A valve arrangement is known which operates without any overlapping operation as disclosed in German Pat. No. 1,478,832. With this valve arrangement a sleeve shaped crossing member sealingly surrounds the cylinder end adjacent the working stroke and the open position exposes an annular gap to the compressed air source, while in the closed position and end face closingly engages a closure member of the outlet valve. Such a valve arrangement is relatively costly, and as the valve member is in the form of a sliding cylindrical valve surrounding the working chamber the driver apparatus necessarily has a long length of height.
Further, a valve arrangement is known in which an elastic retaining lip arranged at the valve piston of a head valve is utilized which is adapted to be biased directly by the pressure of the compressed air source so that the working piston upon movement of the valve piston into the opening position will initially be retained and transported along in a upward direction until, after the outlet valve has been closed, a sufficiently high pressure builds up to release the pistion with high energy so that it may perform its working stroke. This type of valve arrangement works without overlapping in the opening phase because the inlet valve opens only after the outlet has been closed. However, during the closing operation this valve arrangement produces an overlapping operation because the outlet channel is connected to the atmosphere during the entire closing step of the valve piston. Thus, a substantial volume of air may escape directly from the compressed air source through the exhaust port.